1. Field of the Invention
The present invention relates to a twin-shaft multi-lobed type hydraulic device, in particular, such a device used as a compressor for a vehicle air-conditioner or as an air pump.
2. Description of the Related Art
In the prior art, a Roots type compressor and a screw type compressor are well known as twin-shaft and multi-lobed type hydraulic devices. In the Roots type compressor, two mated lobed impellers define three working chambers within a casing; two of these chambers being defined by a combination of the two impellers and the casing, and the third chamber being defined by one impeller alone and the casing. Accordingly, during operation, the volume of the two chambers defined by the two impellers and the casing is reduced, but the volume of the third chamber defined by one impeller alone is not reduced, and therefore, the Roots type compressor cannot provide an effective compression during operation. Further, because there is communication between the chamber having a large volume and the chambers having a reduced volume, during the discharge process, the compressed fluid is slightly decompressed, allowing a fluid flow back to occur between the two chambers, and thus permitting the generation of undesirable noise and vibration. Therefore, a Roots type compressor can be used only for blowing and compression operations at a relatively low compression ratio. On the other hand, although a screw type compressor can provide a high compression ratio, any effective reduction of the size of the unit is limited because the teeth of the screw must be machined to a very high accuracy.